Failures can happen and APT regardless will do a partial cache
update anyway. Because APT ensures that the list directory is
in a sane state, it makes sense to also call success hooks if
success was only partial - otherwise it loses sync with APT.
Most importantly, this causes the appstream cache to be empty,
see launchpad bug #1562733.
This is somewhat overly optimistic though: As soon as any repository
has nonexisting optional files, the missing optional files are also
treated as success, which means a single broken repository without an
InRelease file still runs Success hooks, even though it really should